OBJECTIVE: A randomized, placebo controlled fixed-dose trial was undertaken to determine the efficacy and tolerability of escitalopram in obsessive-compulsive disorder (OCD), using paroxetine as the active reference. RESEARCH DESIGN AND METHODS: A total of 466 adults with OCD from specialized clinical centres, psychiatric hospital departments, psychiatric practices, or general practice were randomized to one of four treatment groups: escitalopram 10 mg/day (n = 116), escitalopram 20 mg/day (n = 116), paroxetine 40 mg/day (n = 119), or placebo (n = 115) for 24 weeks. The primary efficacy endpoint was the mean change in the Yale-Brown Obsessive-Compulsive Scale (Y-BOCS) total score from baseline to week 12. Secondary efficacy endpoints included remission (defined as Y-BOCS total score < or =10), NIMH-OCS, and CGI-S and CGI-I scores at weeks 12 and 24. Tolerability was based on the incidence of adverse events, and on changes in vital signs (blood pressure and pulse). Main outcome measures; RESULTS: Escitalopram 20 mg/day was superior to placebo on the primary and all secondary outcome endpoints, including remission. Escitalopram 10 mg/day and paroxetine 40 mg/day were also effective on the primary scale as well as some other outcome measures. In the escitalopram 20 mg/day group, the improvement in Y-BOCS total score was significantly better than in the placebo group as early as week 6. The most common AEs in the active treatment groups were nausea (19-27%), headache (17-22%), and fatigue (12-19%). More paroxetine-treated patients withdrew due to adverse events than escitalopram- or placebo-treated patients. CONCLUSION: Given that escitalopram 20 mg/day was associated with an earlier onset, higher response and remission rates, improved functioning, and better tolerability than the reference drug, escitalopram deserves to be considered as one of the first-line agents in the pharmacotherapy of OCD for longer-term treatment periods.